This invention relates to low voltage, high current electrical power distribution systems, and more particularly to fuse-protected electrical disconnect devices for use in electrical power distribution systems.
A common problem in the telecommunications industry and in other industries is the distribution of relatively low voltage, relatively high current electrical power to the various devices and equipment which require such power. Telecommunications systems plants, such as telephone switching offices, are typically centralized and are therefore are constructed on a large scale to serve many customers. Since telephone systems must be highly reliable, such plants usually have a plurality of high-capacity storage batteries to provide electrical power for operating equipment whenever power is not available from conventional sources. Often, the batteries are connected in parallel with suitable power conversion equipment which receives electrical power (typically high-voltage AC) from the commercial electric utility and supplies converted power (low-voltage DC) for operating all equipment and for maintaining the batteries in a charged condition. Whenever power from the conventional sources becomes unavailable, power is immediately and automatically supplied from the batteries.
In such systems, several batteries and power converters are usually connected together so that large amounts of equipment are usually supplied with power from a large collection of power sources. It is desirable in such systems to be capable of isolating individual pieces of equipment (or small groups thereof) from the power supply to perform maintenance and installation activities. It is also desirable to provide overload protection for equipment on an individual basis. Accordingly, power is conventionally distributed to equipment from distribution panels having a plurality of individual fused disconnect devices. Each disconnect device controls power to a relatively small load--for example, a cabinet containing subscriber loop interface circuits for 100 subscribers and drawing 10-50 .ANG. in normal operation.
In large installations, multiple levels of power distribution are arrayed such that a single larger fused disconnect is used to distribute power to smaller fused disconnects. Large fused disconnect devices can generally go to 600 .ANG..
In the past, several fused disconnect devices have been developed to allow manual control of each load circuit, and to provide overload and fault protection of each load circuit. Such devices have typically included a line side terminal for a wired connection to a power supply bus, a load side terminal for a wired connection to a load device, a housing, and a removable fuse-containing cartridge which, when installed in the housing, provides an electrical connection between the line side terminal and the load side terminal. Some of these prior art device have also included an indicator fuse disposed in the removable cartridge and connected in parallel with the main fuse. When the main fuse interrupted the circuit, the alarm fuse would complete an subsidiary alarm circuit between the line side terminal and an alarm terminal. The alarm terminal could be monitored for remote indication of a fault.
These prior art devices have a variety of disadvantages. Telecommunications systems are often incrementally expanded. Because customers expect their telecommunications services to be continuously available, it is often necessary to add power circuits in a distribution panel while existing circuits are operating. The rear side of a power distribution panel typically contains uninsulated bus bars and various other uninsulated conductors. Because the panel is supplied with power from batteries and other low-impedance sources, extremely high currents (in the range of 10,000 to 100,000 .ANG.) are available in case of a fault. Installation of disconnect devices from the rear side of the panel may be highly dangerous, since there is a reasonable chance that a conductive tool or part may be accidentally dropped, thereby causing a fault. It is therefore highly desirable to install disconnect devices from the front of the panel.
Generally fused disconnect devices used are not adapted for installation from the front of the power distribution panel. Because of the arrangement of terminals on the prior art disconnects, access from the rear of the panel is generally required.
Another disadvantage of the prior art devices is that their terminal arrangement requires wired connections between the line side terminal and the power supply bus bar. Such wired connections are labor intensive, require expensive parts, and generally require access from the rear of the panel.
Another disadvantage is that in prior art devices providing an alarm fuse, that fuse is contained in the removable fuse-holding disconnect cartridge. Accordingly, if a craftsperson removes the cartridge to disconnect power to a load device, there is no mechanism for generating an alarm signal. Furthermore, when a disconnect fuse has interrupted power to a load device due to an overload or fault condition, merely removing the fuse-holding cartridge is sufficient to defeat the alarm. Accordingly, if a craftsperson removes a cartridge from the prior art disconnect devices and fails to replace it, the equipment served thereby may remain without power for an indeterminate period, and the lack of an alarm signal will conceal that fact from remote monitoring displays.